Vcard based avatars


![serve_avatar_presence001](https://cloud.githubusercontent.com/assets/652428/5222493/57639550-7699-11e4-9724-25c34d195a0d.png)
![serve_avatar_presence002](https://cloud.githubusercontent.com/assets/652428/5222494/57660dbc-7699-11e4-89cc-31977c3873c2.png)

**TODO**:

- [ ] missing unit tests

serve diaspora avatar via presence update related to #43 
mh can we just remove the `rescue`. activerecord should already return `nil` if it finds a `NULL` entry in sql, right?